## Calculating Gas Velocity

## Calculating Gas Velocity

(OP)

Hello,

Is there a quick version or rule of thumb version of calculating gas velocities in a gas well flow back line? There are so many variable, that if changed slightly can really change all of the calculations.

Just curious if someone has a quick and dirty calculation to get approximate values, making some standard assumptions, quickly without going down the rabbit hole of calculations to get very accurate results.

Would like to see what the velocity in a 3" 1502 pipe would be at various flow rates.

For example assuming:

flow rates of

45 MMSCFD

30 MMSCFD

15 MMSCFD

@ Various pressures

1500 psi

1000 psi

750 psi

500 psi

@ 80 Deg F gas temp

@ 0.7 SG of gas

@ 2.625 ID pipe

@ 80 feet long straight pipe

Is there a quick version or rule of thumb version of calculating gas velocities in a gas well flow back line? There are so many variable, that if changed slightly can really change all of the calculations.

Just curious if someone has a quick and dirty calculation to get approximate values, making some standard assumptions, quickly without going down the rabbit hole of calculations to get very accurate results.

Would like to see what the velocity in a 3" 1502 pipe would be at various flow rates.

For example assuming:

flow rates of

45 MMSCFD

30 MMSCFD

15 MMSCFD

@ Various pressures

1500 psi

1000 psi

750 psi

500 psi

@ 80 Deg F gas temp

@ 0.7 SG of gas

@ 2.625 ID pipe

@ 80 feet long straight pipe

## RE: Calculating Gas Velocity

Should not be too difficult to key in data and check the results!

https://checalc.com/fluid_flow_compressible.html

Pierre

## RE: Calculating Gas Velocity

P*(144)*Q = m/sec*R/MW*T*z

and then

P*(144)*Vel*A = m/sec*R/MW*T*z

Where

P is pressure in psia converted to psfa by multiplying by 144

V is volume in cubic feet

Q is flowrate in cubic feet per second

Vel is velocity in feet per second

A is pipe flow area in square feet

m is mass in pounds (weight mass W not W/g mass)

m/sec is mass flow in pounds per second

R is 1545 ft-lb/lb mole-deg Rankin (gas constant)

MW is the molecular weight of gas which is S.G. times 29 (MW for air)

T is temperature deg Rankine

Z is compressibility factor

Then solve the equation for "Vel"

## RE: Calculating Gas Velocity

Work out the velocity at atmospheric conditions for your flow then divide by the pressure in bar. probably within about 15%

Forget temperature - the difference is negligible

Forget density - its volume which matters here.

forget the length - 80 ft is nothing. there will be no real change in velocity from one end to the other unless you're doing hundreds of feet a second. 80 miles yes, 80 ft, no

What's a "flow back line"? Gas lift or gas injection?

that's a pretty thick 3" pipe if your OD is actually 3.5" - sch 160?

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Calculating Gas Velocity

pierreick - I like the online calculator, although I'm not sure how its getting velocity without a flowrate?

Snickster - great information, thank you. I'll rearrange this equation to solve for Vel.

LittleInch - Are you talking about using the equation Snickster posted, only solving for Vel in metric? I'm not so much worried about the change in Velocity, just worried about max velocity and erosion.

Flowback line is a smaller 1502 style line usually 2, 3, 4 inch line. 1502 iron is rated for 15,000 psi. The ID of the 3" 1502 line is 2.625". This is gas flowing back from a well that is being boosted to a collection site.

## RE: Calculating Gas Velocity

## RE: Calculating Gas Velocity

Snickster gave you the definitions. Please read them:

- m is mass in pounds (weight mass W not W/g mass)
- m/sec is mass flow in pounds per second
- MW is the molecular weight of gas which is S.G. times 29 (MW for air)

Z = 1 for NG depends on how close it is to being a liquid, but it should be okay in your case.Good Luck,

Latexman

## RE: Calculating Gas Velocity

## RE: Calculating Gas Velocity

Good Luck,

Latexman

## RE: Calculating Gas Velocity

Don't think so.

Let's break this down then you can make a spread sheet.

1 convert mmsfd to m3/sec. Multiply by 0.33

2 Find [Edit] square area of your pipe ID

(Id/2)^2 x pi. ID in metres

3 velocity at 0barg = (1) / (2)

4 To find velocity at a pressure in bar divide (3) by the pressure.

Answer in m/sec. For ft/ sac multiply by 3.3.

So 45mmscd at 500 psi (worst case) is

4242m/sec divided by pressure.

So 500 psi is approx 35 bar, so a velocity of 121m/sec. Pretty fast.

Accuracy of this method prob about 10% due to using perfect gas law.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Calculating Gas Velocity

LittleInch - That is so helpful, thank you so much for this. One question, if when finding the volume of length of pipe, instead of using 1m length I used 24.384 m (80ft) would that give me the velocity in the full length of the pipe?

Thank you again for all your help.

## RE: Calculating Gas Velocity

No. Can you be specific?

Good Luck,

Latexman

## RE: Calculating Gas Velocity

Sorry, its actually pipe area you're calculating so the calculation (actually just the square area) gives you velocity in m/sec. see EDIT This will be virtually the same at the start of 8ft as it is at the end.

If you mean how long does it take then that's something different

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Calculating Gas Velocity

Here is what I came up with on various pressures and flowrates for velocities. Does this look correct to you? Seems a little high to me.

## RE: Calculating Gas Velocity

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

## RE: Calculating Gas Velocity

## RE: Calculating Gas Velocity

A 3" pipe is pretty small for the highroad flowrates, even at higher pressure.

Below 10 bar the simple method doesn't work as you should be using bara, but that just complicated the formula. You need to round down to the beard 5 or 10 ft/ sec to avoid spurious accuracy. 10% is about as good as it gets here.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Calculating Gas Velocity

Unless this is a vent line doing sonic flow, velocity over 20m is going to be pretty close one end to the other. 20km is very different.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: Calculating Gas Velocity

15 mmscfd

500 psig

80 deg F

0.7 SG

2.625 ID

First find mass flow rate in pounds per second using ideal gas equation at 14.7 psia and 60 deg F

15 mmscfd = 173.6 scfs

P(144)(Q)= m/sec(R/MW)(T)

14.7(144)(173.6) = m/sec(1545/0.7(29))(520)

m/sec = 9.29 lbs/sec

Now use ideal gas equation again at actual conditions and this mass flow rate:

P(144)(Vel)(A)= m/sec (R/MW)(T)

514.7(144)(Vel)((3.14/4)(2.625/12)

^{2}) = (9.29)(1545/0.7(29))(540)Solve for Vel = 137 ft/sec

## RE: Calculating Gas Velocity

To reply to your question, the driving force is the pressure difference. This is a requirement to calculate the mass flowrate thus the volumetric flow rate and the velocity. All the relations are given in the calculator I shared with you.

Additional information about NG is given in the attached document.

Avoid shortcut.

My view

Pierre

## RE: Calculating Gas Velocity

https://www.pipeeng.com/gas_velocity.html

## RE: Calculating Gas Velocity

Pierreick - Thank you for the document giving examples and solutions, this too is very helpful.